Temperature fluctuations affect the mutation that skies have from what I've read. Not sure if it's 100% certain thoughI think that's possible. Just like some chickens hatch with some kind of mutation like extra toes, missing toes. I don't think that affects weather they're pure or not. It's not set in stone that pure silkies will always have 5 toes. One of mine has 4 on one foot and 5 on the other. I've heard it can be the same with nails. I think because of how wierd the 5 toes can be sometimes they are born with a missing nail on their 5th toe.
Basically I don't think that means they're not pure. They just hatched that way.